1. Field of the Invention
The present invention relates generally to semiconductor manufacturing process. More particularly, the present invention relates to probing and testing of dice.
2. Description of the Related Art
Electronics based on semiconductor technologies has become an essential and integral part of modern life over the last few decades. Semiconductor chips containing millions of components are embedded in many electronic devices or machineries, and these semiconductor-based electronic devices are commonly found in many areas of our life, including entertainment, medicine, manufacturing, and transportation.
As these electronic devices become more ubiquitous, the requirements placed on semiconductor chips are getting stricter and broader. Many integrated circuit (IC) chips are, for example, used in modern vehicles (e.g. a passenger car) for various purposes and functions, some of which are crucial for operation of the vehicles. Many of these chips, produced in the same way or from the same process, should be operable in many different and sometimes varying environments such as temperature variations ranging from −50° Celsius to 200° Celsius or sometimes much higher. This requirement on electronic devices, in turn, puts additional requirements on semiconductor manufacturing processes. Among other things, IC chips need to be tested in these different operating environments. For example, it is not uncommon for manufacturers to test these chips at many different temperatures during the manufacturing process, often before packaging.
IC chips are often manufactured on a semiconductor substrate such as a silicon wafer. A semiconductor wafer, typically of a circular shape, usually includes numerous IC devices arranged in a grid pattern. Located on each IC are multiple test or bond pads that are used to connect the IC to external circuitry to allow the IC to function. These IC chips on a wafer, or dice, are often tested using a probe card connected to a testing machine. The probe card has a collection of contact electrodes, or probe pins. The wafer is then positioned, in preparation for testing, so that the pins on the probe card make contact with the die's test or bond pads. This process is known as wafer probing, and a special machine called a wafer prober is used for this purpose. Electroglas, Inc. of San Jose, Calif. is a company which makes wafer probers. In some cases, probing is done on a die or dice which have been cut, or diced, and mounted on other flexible or rigid substrates such as film-frames or “strips”. Electroglas, Inc., for example, also manufactures machines designed for this purpose, often called test handlers. However, many relevant operations of probing and testing in these cases are essentially the same as in the case of probing or testing of (unpackaged) dice on uncut wafers. We will use the term wafer probing, or die probing, to include these more general situations throughout this disclosure.
The primary purpose of wafer probing is to properly position the wafer so that the device's test or bond pads, or simply pads, make sufficient electrical contact with a probe card's probe tips. High accuracy is required because the pads are very small, often of the order of 30˜50 microns. If a probe card pin makes contact outside the pad area the device must be repositioned, slowing down the testing process, and often requiring operator intervention. Moreover, incorrect positioning can cause damage to the device since the probe card pins may break through the passivation layer of the device.
Changing testing conditions or environments in the middle of a wafer probing and testing procedure for a particular wafer or from wafer to wafer is a rather expensive requirement. Once the testing condition is changed, the wafer probing, as well as measurement and calibration, should be redone since probe cards and wafers and other components involved in probing and testing might have different properties and characteristics under different physical conditions. What is more critical is the fact that it takes a while for the system to reach a steady state after change in condition, such as change in the temperature of a wafer chuck. For example, it takes over an hour for a typical wafer prober system to reach a thermal equilibrium after a temperature change of 100° C. or so. This is often referred to as thermal agility of the wafer prober in the related art. If no testing can be done during this period of thermal relaxation, the number of dice that can be tested in a given amount of time will be significantly reduced, thereby adding additional overhead to the production cost per integrated circuit device.
Different materials in the system typically have different susceptibilities and they react differently to changing conditions, further complicating the testing process during the transient period, in which states of the various components within the wafer prober system are changing. For example, when a controlled temperature of a wafer chuck is changed, different parts of a probe card, a wafer, cameras, etc, all expand or contract at different rates and with different amounts.
Present known methods of wafer probing often exhibit inferior performance, especially under dynamically changing conditions such as large changes in temperature. In the current state of the art where probing and testing is done under varying conditions (e.g. at different temperatures), new measurement and calibration is typically done after an environmental change has equilibrated and is performed frequently thereafter until the misalignment shifts are verified to be small. Considering the fact that a measurement and calibration typically takes an order of a minute, frequent measurements can significantly reduce the number of dice that can otherwise be tested in a given time period.
A typical probing and testing process, after change in temperature, in the prior art is shown in FIG. 1A. The flow chart starts at block 102 and ends at 114. Once the temperature of a wafer chuck is changed and a sufficient amount of time has passed 104, a new measurement and calibration needs to be done 106 before probing any further dice. Then, a die or a set of dice is selected and probed based on the previous measurement 108. Once probing is done, desired testing is performed on the selected die or set of dice 110. Then, the process either terminates or continues with another die or set of dice depending on requirements and other conditions 112. In the case where the process continues, the wafer prober needs to be re-calibrated 106 to account for the changing dimensions due to thermal expansion or contraction of various materials. This is because the temperatures of various parts of the prober, including a probe card and a wafer, are constantly changing during the transient period, asymptotically reaching a new temperature that relates to the change in the controlled temperature of the wafer chuck. In this particular scenario shown in the flow chart, if probing and testing of a particular die or dice takes longer than a certain critical duration of time, the testing cannot be done efficiently during the transient period.
When multiple probing and testing are done between each calibration, the parameters from the most recent measurement are used for all subsequent probing before the next measurement. Since a system tends to relax in a roughly exponential fashion and because the thermally-induced misalignment error dynamics is similar to that of a “random walk”, these values will get less and less accurate on average as the time between two consecutive measurements increases. This will further reduce the number dice that can be probed and tested within a given time period. Furthermore, less and less accurate values will be used at or near the end of each measurement and calibration cycle, and it will increase potential for testing errors and further risk damaging the devices being tested.
FIG. 1B shows an example of this process, in the prior art, for probing and testing dice after a change in temperature. This exemplary process can be implemented, for example, using a commercially available wafer prober Horizon 4090® by Electroglas, Inc. The process, defined between terminal blocks 122 and 134, starts from block 124, where the control temperature of a wafer chuck changes from T1 to T2. Once the wafer chuck reaches the desired temperature T2, dimensions of various components of the wafer prober machine, and distances between these components, need to be measured and various measuring gauges and tools need to be calibrated, as shown in block 126. Then the probing 128 is done based on the measured values, and the testing process 130 follows. After the testing is done, the process can either terminate, following the Yes branch at block 132, or it can continue with other dice, following the No branch.
During the transient period (e.g. as the temperatures of various components approach their respective equilibrium or steady-state temperatures), the prober needs to be re-aligned frequently (e.g. once every minute) to account for the changing dimensions due to thermal expansion or contraction of various components. In the flow chart shown in FIG. 1B, the decision as to whether a new measurement is needed is made based on the time elapsed since the last measurement and/or based on the temperature change of a particular component since the last measurement. This is illustrated in blocks 138 and 140 in the figure. At block 138, the elapsed time and/or the temperature changes of relevant parts of the wafer prober are measured. Then, at block 140, these values are compared to preset values. If the elapsed time is longer than a preset duration, or if any of the temperature changes of the relevant components since the last measurement is larger than a certain preset value, then a new measurement is required, and the process continues through the Yes branch at 140. On the other hand, if these criteria are not met, the probing and testing continues, following the No branch at 140, without new offset measurements.
It should be noted that the flow chart of FIG. 1B shows two loops, one going through 132, 138, 140, 128, and 130, which we call the inner loop, and the other going through 132, 138, 140, 126, 128, and 130, which we call the outer loop. Depending on the outcome of the decision block 140, the process can flow through either loop. During the process of testing and probing multiple dice or sets of dice, the more often the inner loop is used, the more dice can be probed and tested. However, the measurement values used for any particular probing will be less and less accurate. On the other hand, the more often the outer loop is traversed, the more accurate the probing will be. However, less number of dice can be probed and tested at the expense of the higher accuracy. Moreover, there is inherent limitation to the accuracy that can be attained in the prior art.